These are essentially boxes that car rental companies provide in order that car hire keys can be left when no one from the car rental company is available, general this means out of hours, so if returning (as an example) to Southampton before 8am you will be required to park the vehicle in one of the car hire bays and post the keys through the letter box provided.

Rob provides a little bit more information as per the below, which says the same as the above but in a different way, again always worth checking the facility is available prior to assuming that it is. We have seen people over the years take keys back to their home countries, which as you can imagine makes things slightly tricky.

A key drop box is basically a box which will be located near to the desk in the airport or near to the city centre branch which is a secure box where you can leave your car hire keys. These are used when you return your vehicle and the branch is either closed or you are returning in unsociable hours. This is quite a handy facility to be aware of and can help you save a little money when it comes to arranging flights. Always check before booking to ensure that a key drop box is a available at the location you wish to hire from.

****************************************************************************************************************************************************************************************It is also worth noting that when the vehicle has been left in the car hire companies parking space, the car is left at the hires risk, and does so until the vehicle has been checked in by one of the car hire attendants to ensure the vehicle is free of damage and has been returned with a fuel tank of fuel or at the pre-agreed fuel level - and please ensure when you are posting the keys you are doing so through the correct rental companies letterbox

At some random locations, suppliers also charge extra for this or simply don't allow it. At airport locations this is always free of charge, but for example in Liverpool at the Avis branch, you either have to return the vehicle back to the airport on the next day it is open by 08:30, or return the vehicle back to the airport by the time it is due to be returned at. Some suppliers charge extra as you have to leave the vehicle in a secure car park, which can cost from £5.00 upwards for an overnight stay.

Recently Europcar at their London offices have removed a large majority of their drop boxes, due to the high risk area in which vehicles are meant to be left, which has unfortunately caused many of their vehicles to become damaged, ruining it for others.